Method for enhancing collection efficiency and providing surface sterilization of an air filter

ABSTRACT

A method of filtering air includes the steps of providing a filter element, providing a pair of electrodes sandwiching the filter element, applying a DC electrostatic field to the electrodes to produce attracting forces between particulates and micro-organisms contained in the air and the filter element, and intermittently applying a sterilizing electrical field concurrently with the electrostatic field. An RF, DC pulse, or AC power supply can be used to generate the sterilizing electrical field.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to air filters, and more particularly tothe air filter having improved capture efficiency of micro-organismscontained in the air; and even more particularly, the present inventionrelates to an air filter having combined electrically enhancedfiltration and surface discharge (plasma) or non-discharge sterilizationfor destruction of the captured micro-organisms.

2. Prior Art

The problem of purification and filtration of indoor air is an importantone and cannot be over-estimated. Tuberculosis, legionella, sinusitis,allergies, bronchitis, asthma, and other health problems can be causedto the large extent by the indoor air pollution. Therefore, airfiltration systems providing an adequate particle removal efficiency areconstantly needed for purification of the indoor air.

Numerous air filters with electronically enhanced capturing capabilityhave been described in the literature and are available in thecommercial marketplace. In some of these systems, the improvement of afilter's capture efficiency is achieved through the application ofelectrostatic fields to a filter. For example, a high efficiencyelectronic air filter is disclosed in U.S. Pat. No. 5,573,577 in whichpads of dielectric fibers are sandwiched between electrically chargedionizing elements, and grounded screens. The ionizing elements chargethe dust particles passing through the filter and at the same time,polarize the fibrous filter pads. In this way, the charged particles areattracted and collected on the fibrous pads with improved efficiency.

As disclosed in U.S. Pat. No. 5,405,434, an electrostatic filter forpurifying air in an EVAC system includes a pair of conductive filamentsinsulated from one another and disposed close together in asubstantially parallel side-by-side relationship. Circuitry is providedfor applying an electrical potential difference between two conductors.The strong electric fields cause the wire sets to attract fine airborneparticulate matter in the vicinity of the filter mesh so that the meshretains dirt, atmospheric ions, and other very fine particles. Suchparticles include pollen and bacteria borne by the air stream passingthrough the mesh which are removed from the air.

U.S. Pat. No. 5,593,476 describes a high efficiency air filtrationapparatus utilizing a fibrous filter medium that is polarized by a highpotential difference which exists between a pair of electrodes. Theelectrodes include an insulated electrode and an uninsulated electrode.A corona precharger is positioned upstream of the electrodes and filter.The corona precharger applies a charge to particles which are removedfrom the air flow system as they accumulate on the filter surfacesproximal to the insulated electrode.

Although filters are good candidates for removing sub-micron airborneparticles (0.3 micrometer diameter particles are captured withefficiency greater than 99%), their capture efficiency however decreasesrapidly for particle diameters below 0.3 microns. This is a majordisadvantage of electrically enhanced high efficiency particulate airfilters since these filters fail to effectively purify the indoor airfrom airborne micro-organisms hazardous to health. The filtration ofbacteria and viruses from indoor air is hindered by two characteristicsof the organism which are the extremely small size of the organisms andthe ability of the organisms to propagate. The typical diameter ofbacteria is a few micrometers, however, viruses can be {fraction(1/100)}th of this diameter. Therefore, it is not only difficult tocapture airborne pathogens on the filter material due to their smalldimension, but also the organisms that are captured by the filter maypropagate on the filter surface and migrate through the filter. Thesecombined factors necessitate frequent filter changes. The control ofairborne pathogens in an indoor environment is especially acute due tothe fact that a major, if not predominant source of airbornemicro-organisms results from entrainment of colonies that have grown onthe filter.

In external environments such as outdoor air, the micro-organisms die asa result of sunlight, temperature extremes, and dehydration. However,for indoor conditions, the range of temperatures is relatively narrowand the air is shielded from direct sunlight. Further indoor air may behumidified, thus aiding in propagation of the organisms. Relative tooutdoor air, the quality of indoor air can be 20-70 times worse. Anexample of the seriousness of biological contamination in indoor air isLegionnaires disease. The legionella bacteria were first discovered in1976 as a result of the Legionnaires disease outbreak in Philadelphiathat caused 200 cases of this disease. Legionella was found to be thecause of a similar outbreak the previous year at the same hotel as wellas a series of mysterious epidemics going back 50 years.

Another example, tuberculosis, is spread via the air through inhalation.Microbacterium tuberculosis is carried in airborne particles known asdroplet nuclei that are generated when persons with pulmonary orlaryngeal tuberculosis sneeze, cough, speak, expectorate, or exhale. Thedroplet nuclei are so small (1-5 micrometers) that they can be suspendedindefinitely in the air and be spread throughout a facility by an HVACsystem. The probability that a susceptible person becomes infected withmicrobacterium tuberculosis depends primarily upon the concentration ofinfectious droplet nuclei in the air and the exposure duration. Unlikeother airborne diseases, which require large aerosolized colonies ofbacteria to produce an infection, one tuberculosis bacillus is enough toinfect humans.

None of the known filter systems, even those with electrically enhancedcapture efficiency appear to prevent micro-organisms propagation andentrainment into the indoor air from the filter.

It is, therefore, clear that the problem of airborne pathogens in indoorenvironments has not been solved by the electrically enhanced filterswhich are known to those skilled in the art. Therefore, a new, improvedair filter is needed which would not only has an increased capturingefficiency, but also provides a biocontaminant control of the indoorenvironment by impeding propagation of the pathogens on the filtersurface and preventing re-entrainment of the organisms into the air fromthe filter.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an airfilter capable of capturing even the smallest airborne organisms anddestroying the organisms thus captured.

It is another object of the present invention to provide an air filterhaving combined electrostatically enhanced filtration performance andhigh sterilization efficiency.

It is still a further object of the present invention to provide aneffective indoor air biocontaminant control by collecting airbornepathogens on a filter and killing them at the place of deposition and invicinity thereof by glow discharge or non-discharge fields.

The present invention may find utility in any air handling system, suchas HVACs, or other systems which displaces and/or distributes air in arelatively closed environment.

In accordance with the present invention, an electrically enhancedfilter includes a filter media towards which air stream laden withmicro-organisms and other particulates is directed. The filter includesa pair of electrodes sandwiching the filter media therebetween havingone or two power supplies coupled to these electrodes. One power supplymay be a DC power supply creating an electrostatic field applied acrossthe filter media between the electrodes and produces attractive forcesbetween the micro-organisms (as well as other particulates in the airstream) and the filter media to enhance filtration efficiency of thefilter. The AC or DC power supply is coupled between the first andsecond electrodes and operate constantly without interruption.

Where a second power supply is applied, such may be RF, DC, pulse, or ACpower supply which is intermittently operationally coupled to theelectrodes for applying intermittent RF electrical field across thefilter media creating periodic uniform gas discharge to destroy themicro-organisms collected by the filter media and in the vicinitythereof.

It is noted that instead of the RF power supply, a non-dischargingelectric field may be used which would create a strong non-discharge“flat” field causing a significant reduction in the number of activemicro-organisms collected in the filter media.

The filter media, preferably is formed of a dielectric material whichmay be a spun fabric dielectric, glass fibers, polypropylene orpolyester, or other like material composition.

Both electrodes are porous electrodes, which allow an airstream to passtherethrough. Preferably, the first electrode includes a plurality ofelectrically conductive (uninsulated) wires interconnected in asubstantially parallel arrangement structurally held together by aninsulated frame. Another electrode includes a plurality of insulatedwires interconnected in a mesh type arrangement which is alsostructurally coupled by an insulated frame.

The power supply is intermittently coupled between the electrodes forpredetermined discharge time sufficient to destroy the micro-organism.For example, exposure times on the order of minutes are applied forkilling a wide range of micro-organisms.

Viewing the subject system from another aspect, the present inventionteaches a method of filtering air from airborne micro-organisms (andother particulates) which includes directing airstream laden withmicro-organisms (and other particulates) towards filtering medium.

A second step applies permanent electrostatic fields across thefiltering medium, thereby enhancing capturing efficiency of thefiltering media. In one embodiment, the invention intermittently appliesa discharging electrical field across the filtering media, whichgenerates periodic uniform gas discharge on the surface of the filteringmedia, or applies a strong non-discharging electrical field, therebydestroying the micro-organisms collected at and in the vicinity of thefiltering media.

In a concluding step, airstream purified from the micro-organisms (andother particulates) is directed from the filtering media.

These and other novel features and advantages of this invention will befully understood from the following detailed description and theaccompanying Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a filter of the present invention;

FIG. 2 is a perspective view showing filter medium electrodesarrangement in the filter of the present invention;

FIG. 3A shows the structure of the downstream electrode of the presentinvention;

FIG. 3B shows the structure of the upstream electrode of the presentinvention;

FIG. 4 shows schematically an experimental set-up for testing the filterefficiency of the filter of the present invention;

FIG. 5 presents a Data Table collected during tests;

FIG. 6 shows schematically the electromicrobicide test array; and,

FIG. 7 is a diagram showing the percent survival of E.coli cells forvarious lengths of exposure to a strong non-discharge DC electric field.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the filter system 10 of the present inventionincludes filter medium 11, a pair of electrodes 12 and 13 sandwichingthe filter medium 11 therebetween, and power sources 14 and 15electrically coupled to thy electrodes 12 and 13. The entire arrangementis housed within a filter housing 16, the wall 17 of which has an input18, and a wall 19 of which has an output 20.

An airstream 23 laden with particulates, such as dust, inorganicparticles, and/or living micro-organisms 22, may be directed by aconventional electrically powered blower (not shown) towards through theinput 18 of the housing 16 towards the filter medium-electrodesarrangement. After filtration, the purified air stream 24 substantiallydevoid of airborne particulates is directed from the filter medium tothe output 20 and is supplied to a facility where it is intended to beused.

Electrode 12 is located downstream of the airstream, while the electrode13 is positioned upstream of the airstream. As shown in FIGS. 2, 3A and3B, electrode 13 is a wire mesh formed of a conductive electrode core(copper or other electrically conductive metal), which is completelycovered or coated with insulation. The insulation may include, forexample, any material having a dielectric constant greater than that ofthe electrode, and materials, such as glass fiber, silicon elastomers,or like material having a high dielectric constant. As best shown inFIGS. 2 and 3B, electrode 13 has an insulating frame 25 surrounding thewire mesh of electrode 13.

Electrode 13 may be insulated by any method known to those skilled inthe art, which may include, for example, dipping or sprayingsimultaneously, extruding or injection molding an insulator with thewire, and/or joining together injection molded insulator sections arounda wire.

Electrode 12, best shown in FIG. 3A, is formed of uninsulated wireswhich are arranged in parallel to each other and framed by insulatedframe 26.

Filter medium 11 may be a commercially available thermostat filter madeof manmade materials such as polypropylene, or polyester, or any otherfibrous filter element, which may be flat or somehow else shaped toincrease the surface area thereof. To simplify the discussion, filtermedium 11, for purposes of example, is a rectangular flat piece ofdielectric fibrous material with dimensions of 1′×1′. The filter medium11 is sandwiched between the electrode 13 and the electrode 12 which areopposedly charged with DC power source 14 and RF power source 15,respectively, both coupled to the electrodes 12 and 13.

The DC power source 14 charges the insulated electrode 13, for instance,with a negative charge, while the uninsulated electrode 12 is chargedwith a positive charge, thereby creating an electrostatic field appliedacross the filter medium 11. The field applied to the electrodes may beas high as 20 kilovolt per inch. The applied field induces apolarization state along the respective lengths of individual fibers ofthe filter medium 11.

In operation, a blower (not shown) moves particle laden incoming air 23through the input 18 and through the pores 27 of the back electrode 13to the filter medium 11.

Therefore, the air passing through the filter medium 11 leaves theparticulates, including micro-organisms 22, on the filter medium 11.Electrostatic fields applied across the filter medium 11 result inoptimized collection efficiency which may be as high as approximately99% for a one micron particle. Either one of the electrodes may beinsulated so as to inhibit spark-over. Any charge may be applied to theelectrodes 12 and 13 so long as they are charged by opposite sign.

The filter medium 11 captures inorganic particulates and livingmicro-organism 22 which may propagate through the filter surface. Themicro-organisms 22 may grow colonies on the filter and further mayre-entrain into the airstream supplied to facilities, therebybiologically contaminating the indoor environment. In order to impedecontamination of the indoor air by micro-organisms collected at andcaptured in the vicinity of the filter medium, the filter medium 11 issterilized in order to destroy the micro-organisms which are depositedat and retained in the vicinity of the filter medium 11. Thesterilization may be performed either by subjecting the micro-organismsto glow discharge or to non-discharge strong electrical fields.

For creating a discharge, the filter 10 of the present invention may beprovided with RF power supply 15 or pulse, AC or DC power supplieswhich, being coupled to the electrodes 12 and 13, may generate a plasmasheet on the surfaces of the filter medium 11 when an AC or pulse poweris applied. The electrodes 12 and 13, positioned on both sides of thedielectric filter sheet 11 and energized with the RF power source 15constitutes a uniform glow discharge plasma reactor. When the dielectricfilter medium 11 captures bacteria and viruses, the plasma produced bythe periodic energization of the electrodes 12 and 13 sterilizes thefilter medium 11 and kills the captured organisms.

By permanently applying a DC, AC, RF, or pulse voltage to the electrodes12 and 13 below the discharge onset (thereby enhancing particle captureacross the filter medium) and by periodically applying voltages from thepower source 15 (thereby sterilizing the filter medium 11), the filter10 of the present invention provides a desired substantially completepurification of the indoor air. In overall flow, the air is directedfrom the filter medium 11 through spaces 29 of the electrode 12, shownin FIGS. 2 and 3, existing between wires 30 of the electrode 12, to theoutput 20 of the filter housing 16.

As previously discussed, in order to create a non-discharge field, powersource 15 shown in FIG. 1 may be either a DC power source, AC powersource, or pulsed power supply.

The sterilization of surfaces of the filter medium 11 through exposureto low temperature gas discharges or to strong non-discharge electricalfields, as above presented, has been demonstrated to be very effective.The combination of electrostatic filter enhancement and plasma filtersterilization applied to a conventional air filter results in aneffective capture and destruction device for even the smallestorganisms.

In order to demonstrate the effectiveness of electric field biologicaldecontamination of indoor air, an experimental set-up 31, shown in FIG.4, was prepared. The experimental set-up 31 includes an electro filter10, a room air intake duct 32 leading to the electro filter 10. A DCpower supply 14 provides a continuous electric field for filtrationenhancement, and a power supply 15 provides plasma sterilization. Theair flow passes through the filter 10 and is subsequently exhausted toan external environment through a fan 33. Micro-organisms 22 areintroduced upstream of the filter 10 by an atomization technique. Themicro-organisms penetrating the filter medium 11 of the filter 10 weremeasured by means of pump and flow control devices 34, and filters 35for taking samples in upstream and downstream portions of the duct 32provided for this purpose. The organism capture and destruction isdetermined by rinsing the filters 35 with subsequent culturing of therinse solution to determine the amount of organisms.

The experiments were focused on two classes of micro-organisms—bacteriaand virus. The reasons for inclusion in the study is their ability toprovide detection of sub-lethal stress and the ability to produceendospores. With regard to sub-lethal stress, bacteria are composed of acell wall, which provides protection from the environment. They includea selectively permeable phospholipid bilayer membrane, and a cytoplasm.Within the cytoplasm is a nucleic acid DNA, referred to as the nucleoid.

Based upon cell wall structure, bacteria are divided into two majorgroups, Gram positive and Gram negative cells. One Gram negative modelorganism, E.coli JM105/pGFP-sigma, has been cloned to carry a greenfluorescent protein, so that when stressed, it fluoresces bright greenupon exposure to UV or blue light, thereby providing detection ofsub-lethal stress. With regard to the ability of bacteria to produceendospores, the microbe S. Aureus will form endospores that cannot bedestroyed easily in response to environmental stress. The spores remaincapable of germination into the vegetative cells for many years.

In the experimental arrangement shown in FIG. 4, the micro-organismswere nebulized into aerosols, and introduced by a nebulizer 36 into theduct 32 upstream of the electric filter 10. The filter efficiency wasmeasured by the use of the downstream compression device 35 and thefiltration rate was measured by the cultivation of the cells ormicro-organisms taken from the filter medium 11.

As discussed in previous paragraphs, the filter 10 was tested with abacteria and a virus. These organisms were exposed to the electricalfield for periods of operation of up to 15 minutes. The sample passedthrough a densely packed fibrous filter at very low velocity such thatparticles as small as 0.01 micrometers diffused to the fibers. Themicro-organisms were then removed from the filter by direct immersion ina known volume of buffer, and plate counts were made.

It was found that ten minutes of plasma exposure destroyed 99.99% ofeither micro-organism. The application of a DC electric field decreasedthe penetration of S. Aureus bacteria through polypropylene by a factorof 10.

A second test program was designed to test the capability of threegeneric modes for producing the electric fields—DC, AC, and pulse DCpower sources, to destroy micro-organisms. The negative DC power supply(a hippotronic capacitive voltage multiplier) provided a variablevoltage between 500 and 10,000 volts and created a field strengthbetween 1,000 and 20,000 volts per inch.

A 60 Hz AC power supply (constructed from a variac and an oil furnacetransformer with an 81:1 ratio) provided variable voltage between 1,500and 10,000 volts to create a field strength between 2,500 and 20,000volts per inch.

A small scale pulse power supply (consisted of hippotronic DC supplyconnected to a variable frequency pulsing device) was built to provide apulsing negative DC supply of 20 to 100 pulses per second at 2,000 to10,000 volts.

The following technique was used to expose E.coli to a strong electricfield:

1. Arrange field plateslin parallel configuration such that even fieldstrength is obtained. This is done by using spacers of the appropriatedistance on either side of the field plates.

2. Connect negative DC to the top plate and ground the bottom plate tomassive earth ground.

3. Decontaminate the chamber plates by swabbing with alcohol.

4. Prepare small (1.5 inches×1.5 inches) squares of fiberglass cloth byheating in an oven to 350 F. for 8 hours. This provides uncontaminatedfilter media for placing the bacteria in the chamber.

5. Place the uncontaminated square of filter media in the chamber.

6. Place another uncontaminated filter square in the control dish.

7. Dose each of the filter cloths with 0.1 microliters of diluted E-colibacteria using a pipette. Select a clean pipette tip for each dosing ofthe filter media. E-coli was previously diluted at 1:10⁷.

8. When dry media is required, measure the resistance of the cloth andthen compare to the resistance of the dosed area. When the dosed area isthe same resistance as the dry cloth begin the electrification.

9. Activate the chamber, using the appropriate power supply for thespecific test, for the test time period, recording the voltage andcurrent readings periodically, as shown in Table 1 of FIG. 5.

10. Record data.

11. Remove the fiberglass cloth from the test chamber and press into thegrowth agar at numerous places taking care not to tear the growth media.

12. Remove the fiberglass cloth from the control Petri dish and pressinto the growth media at numerous places taking care not to tear thegrowth media.

13. Culture th e agar dishes in a warm oven, 80 to 90 degrees F., untilgrowth is complete.

14. Obtain results of bacteria colonization—a milky white indicatesgrowth. Individual colonies will be the size of a pinhead to an eraserhead in diameter.

In this portion of the experiment, humidity and temperature of thechamber of the filter was varied, to measure the sensitivity toenvironmental-conditions on the ability to inactivate micro-organisms.The samples were collected from the filter at intervals of several hoursfor determining viability in order to enable the delineation of thedestruction rate.

For Modified Procedure for reducing colonization counts step numbers11-12 are modified as follows:

After activation of the electric field, place the contaminated cloth ina vessel with 20 ml of sterile H₂O; agitate the vessel for 2 minutes,then draw of 0.1 microliters of the dilute solution. Dose the solutiononto the agar dish and using a glass hockey puck; distribute thesolution over the entire dish.

For Modified Procedure for placing filter media in the field, stepsnumber 1-6 are modified as follows:

A special chamber is made to allow holding of a circular fiberglass 2.75inches diameter filter media. The chamber has 2.5 inch diameter holesallowing the filter to be suspended exactly between the field plates.After the filter has been positioned, the media is dosed and theexperiment carried out as usual. Control filters are only needed toblock changing environmental conditions.

For the procedure described above, the field chamber was made from two2¾″×2¾″×{fraction (5/16)}″ aluminum plates. The plates were drilled andtapped to allow attachment of high voltage cables to each. The plateswere then spaced from one another using ¼″ thick plexiglass sheets. Thesheets were drilled with 2½″ diameter holes to allow placement of thefilter media.

Dosing of the filter media and extracting diluted E-coli wasaccomplished by using a calibrate pipette and pipette tips.

Distribution of the E-coli onto the growth media was done usingsterilized bent glass rods made by bending glass stirring rods over aflame. The conformed glass rods were alcohol washed and flamesterilized.

Referring to FIG. 6, the experiment matrix shows the tests that wereproposed to verify cell inactivation by electric field.

Exposure of E-coli cells to electric fields of both non-discharge anddischarge, greatly reduces, and in the case of discharge fieldscompletely inactivates the cells being tested. This is determined bycounting the number of colonies that were cultured in the agar growthmedia. Table 1 presented in FIG. 5 summarizes the collected experimentdata. In earlier tests the E-coli was not diluted after exposure andlarge quantities were incubated in all of the controls as well as someof the exposures. In tests with a −C designation the E-coli was dilutedafter exposure in order to obtain the cell inactivation efficiencies. DCfields were approximately 20 kV/inch; but slightly lower power levelswere achieved with AC (around 13 Kv/inch). As expected, the pulsed fieldstrengths were much higher, around 50 to 70 kV/inch. All pulsed fieldswere run at 20 pulses per second.

Table 1 is sorted, in order of increasing cell growth, therefore themost effective field properties are listed at the upper portion of Table1.

The data clearly indicates that not only a corona discharge, but also astrong electric field causes a significant reduction in the number ofactive E-coli cells. It is noteworthy that using the non-corona fields,regardless of cell dilution, little evidence of cell inactivationoccurred during the 1 min, 2 min, 5 min, 15 min, or 45 minute exposures.This suggests that there is a time/intensity effect to cell destructionfor non-corona fields. In the subsequent one to three hours of exposure,the reduction in active cells is significant. The graph of the −DC fielddata shown in FIG. 7 illustrates this effect. The field strength used inthe tests was the maximum attainable before sparking would occur.

Referring again to FIG. 7, which shows the percent survival of E.colicells for various lengths of exposure to a 20 KV-inch electric field,specifically, E.coli JM105/pGFP-sigma cells were seeded at knownconcentration onto sterile Whatman filter paper. They were exposed tothe field, washed, and plated asceptically onto agar plates. The plateswere grown at ambient temperature for two days. Test samples werecompared to unexposed control cells. All experiments were performed induplicate. It was found that 100% kill of this E.coli was accomplishedin less than 72 hours.

As discussed in previous paragraphs, the electrically enhanced filterhaving the high capturing efficiency and high micro-organism destructionefficiency (99%+) was developed and demonstrated. As the result of testsperformed, it was concluded that

a) living micro-organisms can be deactivated using either a coronadischarge or non-discharge strong electric fields, and

b) corona discharge is not necessary to inactivate the micro-organismsif a long enough exposure to a strong DC field is allowed.

The successful demonstration of the proposed concept and device allowsextremely efficient micro-organisms control for indoor air. Theelectrically enhanced filter improves the collection of sub-micronorganisms (and inorganic particles) by an order of magnitude over someprior art, and thus, is capable of controlling indoor air viruses, aswell as bacterial pathogens. The commercial market for such a filterdevice clearly includes those areas that are traditionally highlysusceptible to airborne pathogens, such as hospitals and other medicalfacilities. It can also include any other facility for which effectivecontrol of airborne pathogens would lead to the improved health ofoccupants such as schools and offices.

In addition, the device of the present invention provides improved fineparticle control for any facility intended for micro-electronicfabrication. The proposed device can directly replace the highefficiency particulate air conventional filters without modifying theduct work or any other component of the HVAC system. It can be designedto use the same mounting hardware now used for high efficiencyparticulate air filters only a small electrical power supply and itscontrol would be needed to be added to the system.

Although this invention has been described in connection with specificforms and embodiments thereof, it will be appreciated that variousmodifications other than those discussed above may be resorted towithout departing from the spirit or scope of the invention. Forexample, equivalent elements may be substituted for those specificallyshown and described. Certain features may be used independently of otherfeatures, and in certain cases, particular location of elements may bereversed or interposed, all without departing from the spirit or scopeof the invention as defined in the appended Claims.

What is claimed is:
 1. A method of filtering air from airbornemicro-organisms, comprising the steps of: providing a filter medium,providing a pair of electrodes sandwiching said filter mediumtherebetween, directing an air stream laden with micro-organisms towardssaid filter medium, continuously applying an electrostatic field betweensaid electrodes, thereby enhancing capturing efficiency of said filtermedium, intermittently applying a sterilizing electrical field betweensaid electrodes concurrently with said electrostatic field, saidintermittent sterilizing electrical field being of sufficient magnitudeto generate a plasma sheet on surfaces of said filter media, therebydestroying said micro-organisms collected at said filter medium and in avicinity thereof, and directing an air stream purified from saidmicro-organisms from said filter medium.
 2. The method of claim 1,wherein said sterilizing electrical field is formed by an RF electricalfield.
 3. The method of claim 1, wherein said sterilizing electricalfield is formed by an AC electrical field.
 4. The method of claim 1,wherein said sterilizing electrical field is formed by a DC electricalfield.
 5. The method of claim 4, wherein said DC electrical field is apulsed electrical field.
 6. A method of filtering air from airbornemicro-organisms, comprising the steps of: providing a filter medium,providing a pair of electrodes sandwiching said filter mediumtherebetween, directing air stream laden with micro-organisms towardssaid filter medium, continuously applying an electrostatic field betweensaid electrodes, thereby enhancing capturing efficiency of said filtermedium, intermittently applying a non-discharging electrical fieldbetween said electrodes concurrently with said electrostatic field, eachapplication of said non-discharging electrical field being for aduration sufficient have a sterilizing effect and thereby destroy saidmicro-organisms collected at said filter medium and in vicinity thereofand further enhancing said capture efficiency of said filter medium, anddirecting air stream purified from said micro-organisms from said filtermedium.